In the oil and gas industry, geophysical survey techniques are commonly used to aid in the search for and evaluation of subterranean hydrocarbon or other mineral deposits. Generally, a seismic energy source, or “seismic source,” generates a seismic signal that propagates into the earth and is partially reflected by subsurface seismic interfaces between underground formations having different acoustic impedances. The reflections are recorded by seismic detectors, or “receivers,” located at or near the surface of the earth, in a body of water, or at known depths in boreholes, and the resulting seismic data can be processed to yield information relating to the location and physical properties of the subsurface formations. Seismic data acquisition and processing generates a profile, or image, of the geophysical structure under the earth's surface. While this profile may not directly show the location for oil and gas reservoirs, those trained in the field can use such profiles to more accurately predict the location of oil and gas, and thus reduce the chance of drilling a non-productive well.
Various sources of seismic energy have been used to impart the seismic waves into the earth. Such sources have included two general types: 1) impulsive energy sources and 2) seismic vibrator sources. The first type of geophysical prospecting utilizes an impulsive energy source, such as dynamite, a mud gun, or a marine air gun, to generate the seismic signal. With an impulsive energy source, a large amount of energy is injected into the earth in a very short period of time. In the second type of geophysical prospecting, a vibrator is used to propagate energy signals over an extended period of time, as opposed to the near instantaneous energy provided by impulsive sources.
The seismic process employing such use of a seismic vibrator, sometimes referred to as “vibroseis,” propagates energy signals into the earth over an extended period of time or “sweep.” In such instances, energy at a starting frequency is first imparted into the earth, and the vibration frequency changes over the sweep interval at some rate until the stopping frequency is reached at the end of the interval. The difference between the starting and stopping frequencies of the sweep generator is known as the “sweep frequency range,” and the amount of time used to sweep through those frequencies is known as the “sweep length.” The recorded data may then be correlated with the sweep signal to produce an interpretable record which resembles a conventional seismic record like one from an impulsive source. In land-based implementations, the seismic source signal is generally generated by a servo-controlled hydraulic vibrator, or “shaker unit,” mounted on a mobile base unit. In marine implementations, vibrators typically include a bell-shaped housing with a large piston or a diaphragm in its open end. The vibrator is lowered into the water from a marine survey vessel, and the diaphragm is vibrated by a hydraulic drive system similar to that used in a land vibrator.
A seismic signal may be also generated by a SEISMOVIE™ system designed and manufactured by CGG Services SA (Massy, France). A SEISMOVIE™ system may emit energy at individual frequencies, one-by-one, until approximately the entire frequency band is emitted. While a SEISMOVIE™ system does not perform a sweep, a frequency band from the starting frequency to the stopping frequency may still be emitted to create an essentially complete discrete frequency dataset. Except where expressly stated herein, “seismic source” is intended to encompass any seismic source implementation, both impulse and vibratory, including any dry land, transition zone, or marine implementations thereof.
The seismic signal is emitted in the form of a wave that is reflected off interfaces between geological layers. The reflected waves are received by an array of geophones, or receivers, located at or near the earth's surface, which convert the displacement of the ground resulting from the propagation of the waves into an electrical signal recorded by means of recording equipment. The receivers typically receive data during the seismic source's sweep interval and during a subsequent “listening” interval. The receivers record the particle motion or pressure in the medium (for example soil, rock, or water) at their location. The received signals can be processed to estimate the travel time from the seismic source to the receiver. Travel time, in combination with velocity information, can be used to reconstruct the path of the waves to create an image of the subsurface.
A large amount of data may be received by the receivers and the received signals may be recorded and subjected to signal processing before the data is ready for interpretation. The recorded seismic data may be processed to yield information relating to the location of the subsurface reflectors and the physical properties of the subsurface formations. That information is then used to generate an image of the subsurface.
In some implementations, seismic sources may emit frequency sweeps or monofrequencies where many frequencies are emitted one by one over a period of time. The emission of frequencies one by one, either in a frequency sweep or a set of monofrequencies, can be time consuming and can reduce the productivity of the seismic acquisition. While impulsive seismic sources may emit many frequencies simultaneously in a single pulse, the phase and amplitude of the frequencies emitted during the pulse may not be controlled thus resulting in the emission of many frequencies that do not produce useable data. Additionally, in some locations, the use of impulsive seismic sources may be prohibited or subject to permitting requirements or other restrictions. Accordingly, it would be advantageous for a vibratory seismic source to emit multiple frequencies simultaneously while controlling the phase and amplitude of the emitted frequencies.